In recent years the use of LEDs in consumer lighting devices has significantly increased as a consequence of the potential for increased service life and increased energy efficiency over conventional fluorescent and incandescent bulbs. In comparison to these types of bulbs, however, a significant amount of heat is produced by LEDs. This heat, if not removed from the LED, will increase the junction temperature, i.e. the temperature at which the LED operates. This has deleterious effects on the efficiency and service life of the LED, as well as the consistency over time of the colour of the light output from the LED. Accordingly, it is important to reduce the junction temperature of the LED as much as possible by removing heat from the LED in operation.
In particular, high brightness LED arrays can be manufactured which include over a hundred LED die in a single package with a light emitting area of only a few cm2. While this small light emitting surface area is highly beneficial in terms of the uniformity of the emitted light, it results in a high amount of heat production, concentrated to a small area, which can result in a rapid increase in junction temperature if not suitably managed.
It is well known in the art to use heat-sinks formed of thermally conducting materials such as aluminium, copper or other metals. Generally, an LED might be mounted on a solid block, from which extends a plurality of fins. These increase the surface area of the heat-sink to allow more heat to be dissipated into the surrounding air by convection. For high-brightness LEDs where the heat output may be in excess of tens of Watts, forced air cooling is often used, wherein fans, piezoelectric microblowers or similar are used to increase airflow over the heat-sink. However, the inclusion of these components in a lighting fixture will increases its cost, complexity and power consumption. Furthermore, the inclusion of such components will increase the noise contamination caused by the fixture and increase the fixture's maintenance requirements.
It is also known in the art to create more complex passive cooling circuits which combine multiple heat-sinks by means of heat pipe technology. For example, WO 2011/032554 A1 is directed towards a cooling device for a heat source, especially LED modules, wherein the LED module is connected to a first heat-sink which comprises a main metal block from which fins extend. This first heat-sink is thermally connected to a second, larger heat-sink by means of heat pipes running through the main block of the first heat-sink.
Fundamentally, however, the removal of heat from the LED module in cooling circuits such as this is limited by the first heat-sink. In particular, any inefficiencies in the transfer of heat from the LED module to the first heat-sink can act as a ‘thermal bottleneck’, resulting in an increase in the junction temperature.
The present invention aims to ameliorate these and other deficiencies of the prior art.